Location: Sheboygan, WIAccident Number: CEN18LA115Date & Time: 03/04/2018, 2324 CSTRegistration: N1174UAircraft: MESSERSCHMITT-BOLKOW-BLOHM BK 117 B-2Aircraft Damage: SubstantialDefining Event: Flight control sys malf/failInjuries: 3 NoneFlight Conducted Under: Part 135: Air Taxi & Commuter - Non-scheduled - Air Medical (Medical Emergency) Analysis Before departure on the air ambulance flight, standard flight control and hydraulic transfer checks were performed with no anomalies noted with the flight controls. The helicopter’s hover, departure, and cruise were normal. During his approach to the hospital, the pilot started a slow descent using the collective and performed the before landing checks; all systems were normal. However, when attempting to decelerate from cruise speed, the pilot noticed restricted cyclic control movement, about 1 inch in any direction, with normal collective and yaw control movement. He then chose to abort the descent and execute a precautionary landing at a nearby airport.During the short flight to the airport, the pilot cycled the hydraulic test switch twice with no change noted to his flight controls. During the approach to land, the pilot held the cyclic trim switch aft in an attempt to gain additional aft cyclic authority. With continuous cyclic pressure against the aft stop, the pilot was able to slow the helicopter to 80 knots on final approach. The helicopter touched down near the runway centerline, skidded about 500 ft, exited the runway surface, and came to rest upright. The helicopter sustained substantial damage to the main rotor blades and fuselage structure.Postaccident examination of the helicopter’s flight control system, including the trim system, did not reveal any evidence of malfunctions or failures that would have precluded normal operations. The cyclic trim was found in the full forward and left position. The flight control dual hydraulic pack was functionally tested on the helicopter and at a repair facility with no anomalies noted. Thus, the reason for the restricted cyclic control movement could not be determined. Probable Cause and FindingsThe National Transportation Safety Board determines the probable cause(s) of this accident to be:The restricted cyclic control movement for reasons that could not be determined because postaccident examination and testing of the cyclic control system did not reveal any evidence of mechanical malfunctions or failures that would have precluded normal operation.FindingsNot determinedNot determined - Unknown/Not determined (Cause)Factual Information On March 4, 2018, about 2324 central standard time, a Messerschmitt-Bolkow-Blohm BK 117 B-2 helicopter, N1174U, operated by Air Methods Corporation, Englewood, Colorado, sustained substantial damage during a hard landing at Sheboygan County Memorial Airport (SBM), Sheboygan, Wisconsin. The commercial pilot, flight nurse, and flight paramedic were not injured. The flight was operated under the provisions of Title 14 Code of Federal Regulations Part 135 as a helicopter air ambulance flight. Night visual meteorological conditions prevailed at the time of the accident, and a flight plan was not filed for the flight that departed about 2257, from Fond du Lac, Wisconsin. The flight was destined for the HSHS St. Nicholas Hospital, Sheboygan, Wisconsin.According to the pilot, prior to departure from Fond du Lac, standard flight control and hydraulic transfer checks were performed, and no anomalies noted with the flight controls; the hover and departure were normal. The pilot, who was wearing night vision goggles, climbed the helicopter to 2,500 ft above ground level (agl) and accelerated to 125 knots indicated airspeed. Approaching the hospital, the pilot started a slow descent utilizing the collective and performed the before landing checks; all systems were normal. When attempting to decelerate from cruise speed, the pilot noticed restricted cyclic control movement, about 1 inch in any direction, with normal collective and yaw control. He then elected to abort the descent and execute a precautionary landing at SBM, located about 6 miles west of the hospital.During the short flight to SBM, the pilot cycled the hydraulic test switch twice, which was located on the cockpit overhead panel, with no change noted to his flight controls. In addition, he asked the flight paramedic, who was seated in the left seat, to check if any circuit breakers were popped, verify systems were in normal operational ranges, and no cautions or advisory lights were illuminated. All circuit breakers and systems were normal with no anomalies noted.The pilot initially flew an approach to runway 22; however, due to being too high and fast, he elected to circle to land on runway 13, which provided a more favorable quartering headwind. During the runway 22 approach and circle to land to on runway 13, the pilot held the cyclic trim switch aft in an attempt to gain additional aft cyclic authority. With continuous cyclic pressure against the aft stop, the pilot was able to slow the helicopter to 80 kts on final approach. The helicopter touched down near runway centerline, skidded about 500 ft, exited the runway surface, and came to rest upright.During the landing, the helicopter's top wire strike protection system was separated due to contact by the main rotor blades, and the landing gear skids were partially collapsed. One main rotor blade had a portion of the blade skin and core separated, and all the blades sustained leading edge damage.Postaccident examination of the helicopter's showed the cyclic trim position was found in the full forward and left position. Examination of the flight control system, to include the trim system, revealed no anomalies that would have precluded normal operations. The flight control dual hydraulic pack was functionally tested on the helicopter with no anomalies noted. The pack was removed from the helicopter for further examination and testing.On April 5, 2018, under the supervision of a Federal Aviation Administration (FAA) inspector, the helicopter cyclic control trim system switch and motors were tested about 50 times in each direction of full travel and from the neutral positions. No anomalies were noted during the tests.On April 16, 2018, under the supervision of FAA inspectors at Air Methods Corporation, which was an approved component overhaul facility, the dual hydraulic pack was tested in accordance with repair manual REM 401-04-22 Dual Hyd-Pack Final Work/Test procedure. No anomalies were noted during the examination or test procedures.A review of the aircraft maintenance logbook revealed no reports of control problems or recent maintenance to the flight control system.According to the helicopter flight manual should a hydraulic system caution light illuminate, the hydraulic test switch should not be operated in-flight, but remain in the [normal] position.Pilot InformationCertificate: CommercialAge: 57, MaleAirplane Rating(s): Single-engine LandSeat Occupied: RightOther Aircraft Rating(s): HelicopterRestraint Used: 4-pointInstrument Rating(s): HelicopterSecond Pilot Present: NoInstructor Rating(s): NoneToxicology Performed: NoMedical Certification: Class 2 With Waivers/LimitationsLast FAA Medical Exam: 03/27/2017Occupational Pilot: YesLast Flight Review or Equivalent: 06/05/2017Flight Time: 5684 hours (Total, all aircraft), 84 hours (Total, this make and model), 3746 hours (Pilot In Command, all aircraft), 18 hours (Last 90 days, all aircraft), 8 hours (Last 30 days, all aircraft), 1 hours (Last 24 hours, all aircraft) Aircraft and Owner/Operator InformationAircraft Make: MESSERSCHMITT-BOLKOW-BLOHMRegistration: N1174UModel/Series: BK 117 B-2Aircraft Category: HelicopterYear of Manufacture: 1988Amateur Built: NoAirworthiness Certificate: TransportSerial Number: 7146Landing Gear Type: SkidSeats: 7Date/Type of Last Inspection: 02/15/2018, AAIPCertified Max Gross Wt.: 7385 lbsTime Since Last Inspection:Engines: 2 Turbo ShaftAirframe Total Time: 8613 Hours as of last inspectionEngine Manufacturer: HoneywellELT: C126 installed, not activatedEngine Model/Series: LTS-101Registered Owner: MILWAUKEE REGIONAL MEDICAL CENTER INCRated Power: 675 hpOperator: Air Methods CorporationOperating Certificate(s) Held: On-demand Air Taxi (135)Meteorological Information and Flight PlanConditions at Accident Site: Visual ConditionsCondition of Light: NightObservation Facility, Elevation: SBM, 755 ft mslDistance from Accident Site: 0 Nautical MilesObservation Time: 2353 CSTDirection from Accident Site:Lowest Cloud Condition: ClearVisibility: 10 MilesLowest Ceiling: NoneVisibility (RVR):Wind Speed/Gusts: 9 knots / 18 knotsTurbulence Type Forecast/Actual: / NoneWind Direction: 110°Turbulence Severity Forecast/Actual: /Altimeter Setting: 30.34 inches HgTemperature/Dew Point: 2°C / -4°CPrecipitation and Obscuration: No PrecipitationDeparture Point: Fond du Lac, WI (FLD)Type of Flight Plan Filed: NoneDestination: Sheboygan, WI (SBM)Type of Clearance: VFRDeparture Time: 2257 CSTType of Airspace: Airport InformationAirport: Sheboygan County Memorial (SBM)Runway Surface Type: AsphaltAirport Elevation: 755 ftRunway Surface Condition: DryRunway Used: 13IFR Approach: NoneRunway Length/Width: 5002 ft / 75 ftVFR Approach/Landing: Precautionary Landing; Straight-inWreckage and Impact InformationCrew Injuries: 1 NoneAircraft Damage: SubstantialPassenger Injuries: 2 NoneAircraft Fire: NoneGround Injuries: N/AAircraft Explosion: NoneTotal Injuries: 3 NoneLatitude, Longitude: 43.764444, -87.851389 (est)